03 Nov 2021
03 Nov 2021
Status: a revised version of this preprint is currently under review for the journal BG.

Biological Nitrogen Fixation in CMIP6 Models

Taraka Davies-Barnard1,2, Sönke Zaehle2, and Pierre Friedlingstein1,3 Taraka Davies-Barnard et al.
  • 1College of Engineering, Maths, and Physical Sciences, University of Exeter, Exeter, UK
  • 2Max Planck Institute for Biogeochemistry, Jena, Germany
  • 3Laboratoire de Meteorologie Dynamique, Institut Pierre-Simon Laplace, CNRS-ENS-UPMC-X, Departement de Geosciences, Ecole Normale Superieure, 24 rue Lhomond, 75005 Paris, France

Abstract. Biological nitrogen fixation is the main source of new nitrogen into natural terrestrial ecosystems and consequently in the nitrogen cycle in many earth system models. Representation of biological nitrogen fixation varies, and because of the tight coupling between the carbon and nitrogen cycles, previous studies have shown this affects net primary productivity. Here we present the first assessment of the performance of biological nitrogen fixation in models contributing to CMIP6 compared to observed and observation-constrained estimates of biological nitrogen fixation. We find that 9/10 models represent global total biological nitrogen fixation within the uncertainty of recent global estimates. However, 6/10 models overestimate the amount of fixation in the tropics, and therefore the extent of the latitudinal gradient in the global distribution. For the SSP3-7.0 scenario of future climate change, models project increases in fixation over the 21st century of up to 80 %. However, while the historical range of biological nitrogen fixation amongst models is large (up to 140 kg ha−1 yr−1 at the grid cell level and 43–208 TgN yr−1 globally) this does not have explanatory power for variations in net primary productivity or the coupled nitrogen-carbon cycle. Models with shared structures can have significant variations in both biological nitrogen fixation and other parts of the nitrogen cycle without differing in their net primary productivity. This points to systematic challenges in carbon-nitrogen model structures.

Taraka Davies-Barnard et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-290', Anonymous Referee #1, 06 Dec 2021
  • RC2: 'Comment on bg-2021-290', Anonymous Referee #2, 14 Mar 2022

Taraka Davies-Barnard et al.

Taraka Davies-Barnard et al.


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Short summary
Biological nitrogen fixation is the largest natural input of new nitrogen onto land. Earth System Models mainly represent global total terrestrial biological nitrogen fixation within observational uncertainties, but overestimate tropical fixation. The model range of increase in biological nitrogen fixation increase in the SSP3-7.0 scenario is 3 to 87 %. While biological nitrogen fixation is an key source of new nitrogen, its predictive power for net primary productivity in models is limited.